The RecA protein of E. coli has been shown to mediate genetic recombination, regulate its own synthesis, control the expression of other genes, act as a specific protease, form a helical polymer and have an ATPase activity, among other observed properties. Understanding the function of the RecA protein will reveal basic mechanisms which are at the foundation of general genetic recombination. Recombination at this moment is assuming an importance far greater than just pure biology. The association between chromosomal rearrangements and neoplasms has become stronger and stronger, and these rearrangements are most likely products of the recombinatory apparatus of the normal cell. Further, damage to DNA appears to be a major cause of cancer. It therefore assumes great clinical significance to understand the various mechanisms available to cells for the restoration of the integrity of their genetic material. Postreplication repair in prokaryotes corresponds to the filling of daughter strand gaps created by the arrest of replication at or near a lesion. Postreplication repair in E. coli is intimately associated with recombination and is dependent upon the RecA protein. Thus, studies of RecA polymers can be expected to help elucidate biological processes which range from meiosis to neoplastic transformation. Structural studies of RecA can make a large contribution towards such an understanding of function. Specifically, the RecA polymer has been observed in several different conformations. These structural states will be related to the enzymatic states of the molecule using a combination of electron microscopy, computed image analysis and biochemistry. Electron micrographs of different complexes of RecA and DNA will be obtained, and reconstructed in three-dimensions using computed image analysis. The position of different DNA molecules in the RecA helix will be determined. Models for the RecA-mediated pairing and strand exchange for two homologous DNA molecules will be developed based upon the location of different DNA molecules in the RecA complex. One obstacle in obtaining a better picture of the RecA filament has been the disorder caused by the steady-state hydrolysis of bound nucleoside triphosphates. The use of ATP analogs which bind to RecA and which cannot by hydrolyzed by RecA will lead to a higher resolution picture of the RecA polymer and the component subunits. A comparison of structures formed in the absence of nucleotide hydrolysis with those formed under the steady-state hydrolysis of the nucleotide cofactor will help reveal the mechano-chemical activity of RecA during the strand exchange reaction, where the nucleotide cofactor is being hydrolyzed rapidly. These studies will provide a structural framework in which basic principles of general genetic recombination can be understood.